The broad objectives of the proposed research are to obtain additional and significant information relative to the growth and morphogenesis of the cellular slime molds. Attention will be given to improving and refining techniques for cultivating and observing these organisms in which the growth and morphogenetic phases of development are separate and sequential to a singular degree. We shall continue to isolate and investigate different genera and species with reference to unique or contrasting patterns of developmental behavior, including: myxamoebal growth, cell aggregation, progressive cellular interdependence in the pseudoplasmodium, and lastly the controlled but divergent differentiation of cells that characterizes culmination, or sorocarp construction. We shall attempt to analyze the beneficial effect of activated charcoal upon the fruiting of certain cellular slime molds, and if possible identify the nature of the gaseous inhibitor(s) that are removed in its presence. Special attention will be given to the macrocysts, structures now known to be produced in four species of Dictyostelium and two species of Polysphondylium. Whereas the macrocysts have been regarded previously as a morphogenetic stage alternative to sorocarp formation, limited ultrastructural and genetic evidence of recent data points to a role of far greater significance, to wit, that the macrocysts are the sites of true sexuality in the cellular slime molds. If this role is fully confirmed, as seems likely, these simple enkaryotic organisms, already being studied in many laboratories for many purposes, should become especially attractive subjects for genetic analyses and manipulation as well. Serious problems still remain, particularly with regard to the maturation and germination of macrocysts produced in cultures by paired mating types of Dictyostelium discoideum. Resolution of these difficulties will be a major objective of our research during the next three years.